igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs). Color: Dark grey or greenish-black host rock with lighter tan or cream-colored mineral fillings. Luster: Dull/earthy for the matrix, vitreous for the amygdules. Structure: Vesicular/aphanitic.
- Hardness
- 6 (Mohs)
- Luster
- Dull/earthy for the matrix, vitreous for the amygdules
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Physical properties
Hardness: 6 (Mohs). Color: Dark grey or greenish-black host rock with lighter tan or cream-colored mineral fillings. Luster: Dull/earthy for the matrix, vitreous for the amygdules. Structure: Vesicular/aphanitic.
Formation & geological history
Formed from the rapid cooling of basaltic lava flows. Gas bubbles (vesicles) were trapped in the lava as it solidified; later, mineral-rich groundwater permeated the rock, filling these voids with secondary minerals like quartz, calcite, or epidote.
Uses & applications
Used primarily as construction aggregate, road base, and occasionally as a decorative semi-precious stone when polished as cabochons.
Geological facts
The name comes from the Latin 'amygdala', meaning almond, because the mineral fillings in the rock are often almond-shaped.
Field identification & locations
Identify by looking for a fine-grained, dark volcanic matrix containing distinct 'spots' or 'pips' of a different mineral. It is commonly found in ancient lava fields like the Lake Superior region or the Deccan Traps.
